Henry’s Fork

Henry’s Fork, April 12th, 2022

We are in a period of “BWO” weather. If you can stand the accompanying wind and precipitation, afternoon top water fly fishing can be worth a visit along the lower river. . Dress warmly and wade carefully! Banks are clearing making access an improving situation. Presenting streamer and stone fly nymph patterns will also bring action during this period of relatively good overhead cover.

Excerpts from Dr. Rob Van Kirk Henry’s Fork drainage SWE report of this morning are below. It contains some long awaited good news.

Headlines:  

• Yesterday was the wettest since January 6, increasing water-year precipitation to 80% of average and SWE to 67% of average.
• Natural flow dropped yesterday in response to cold temperatures.
• Due in part to direct precipitation, Island Park Reservoir gained 444 ac-ft yesterday and is 93% full, compared with 86% on average.
• Cold, unsettled weather is expected to continue for the rest of the week.

Details:

As mentioned yesterday, we could have used our current weather back in February. Mean temperature yesterday was 9 degrees F below average and roughly equal to average at the end of February. Precipitation overachieved for a change and was the highest one-day total since January 6, the last day of the only extended wet period we had all winter. Precipitation totals ranged from 0.11 inch at Rexburg to 0.8 inch at White Elephant and averaged 0.38 inch over the watershed. This was enough to move the water-year total from 78% of average to 80% of average.

Cold temperatures put a temporary end to snowmelt, and all SnoTel stations except Island Park gained snow water equivalent (SWE) yesterday. New SWE ranged from 0.3 inch at Crab Creek to 1.3 inches at Phillips Bench and averaged 0.6 inch over the watershed. This moved SWE up two percentage points to 67% of average. It also moved SWE back ahead of 2001 and 2015, the two years with lowest SWE in the 1989-2022 record. SWE on the ground this morning is the highest for the season so far at all high-elevation stations and most of the mid-elevation stations. Date of peak SWE for the whole watershed is still March 22, but tomorrow’s SWE will most likely be the new peak for the season. Average peak date is April 12.

Very cold and unsettled weather will continue for another few days. Although temperatures will warm, precipitation remains in the forecast off and on into the beginning of next week. The 7-day quantitative forecast calls for 0.25 inch along the western edge of the watershed, 0.5-0.75 inch in the Island Park area and up to 1.75 inches along the Wyoming border. That will be enough to keep the water-year total up above 80% of average and is likely to push SWE up to 70% of average. Snowmelt will remain minimal at least through Friday.

After a small, short-duration bump, natural streamflow dropped yesterday in response to cold temperatures over the past two days. Natural flow is 71% of average: 76% in upper Henry’s Fork, 68% in Fall River, and 62% in Teton River. I expect natural flow to stay pretty constant for the next few days.

Due in part to direct precipitation on the reservoir surface, Island Park Reservoir gained 444 ac-ft yesterday to each 93% full, compared with 86% full on average. Outflow will likely be increased again to prevent reservoir ice from encroaching on spillway infrastructure as the reservoir continues to fill.  

Henry’s Fork, March 15th, 2022

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage water report of this morning are below.

 Headlines:  

• Water-year precipitation stayed at 83% of average yesterday, while SWE dropped to 69% of average.
• Stream gages are starting to come back on line after a very long period of ice, showing natural flow in the range of 75-80% of average.
• Temperatures and precipitation are expected to stay near climatic averages for the next week.

Details:

Mean temperature yesterday was 2 degrees F below average, extending a streak of nine consecutive days of temperatures at or below average. With snowpack as low as it is, cold weather is critical to keeping that snow on the ground as far into the spring as possible. After a dry day, snow water equivalent (SWE) dropped back to 69% of average yesterday, while water-year precipitation stayed at 83% of average.

Current SWE is still ahead of that for the date in 2001 and 2010. Subsequent spring and summer weather differed substantially between those two years, resulting in two very different outcomes for the water year as a whole. April-September precipitation in 2001 was 21% below average, while that in 2010 was 19% above average. That difference resulted in 2001 ending up with the lowest natural streamflow in the 1978-2021 record, at 73% of average, compared with 75% of average in 2016 and 76% of average last year. By contrast, natural flow in 2010 ended up at 87% of average, ranking above the 33^rd percentile of water years since 1978. So, a wide range of outcomes is still possible, although anything close to average water supply is not one of them.

Light precipitation is expected this afternoon through tomorrow and again on Saturday night and Sunday. Total precipitation amounts over the next week are expected to range from around 0.1 inch along the western edge of the watershed to around 1 inch along the Wyoming border. If those totals are realized, precipitation this week will be about the same as it was last week and close to average for mid-March. Temperatures are expected to stay near average. After that, above-average temperatures are in the forecast beginning next Tuesday, although long-range outlooks do not show any strong indication that either temperature or precipitation will deviate much from average.

Stream gages are starting to come back on line after a very long period during which ice has prevented real-time flow observations. The St. Anthony gage on the Henry’s Fork and Ashton gage on Fall River are the first two to return to real-time transmission. Based on data available as of yesterday, natural streamflow is in the range of 75-80% of average in both the upper Henry’s Fork and Fall River.

Island Park Reservoir gained 232 ac-ft yesterday and is 87% full, compared with 80% full on average. Reservoir content will reach 120,000-123,000 ac-ft by April 1.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

HFF blog

Henry’s Fork, March 12, 2022

Nicer but unsettled weather is predicted for the coming week. With increasing BWO activity on the lower river, some decent top water fishing should take place but river still has limited approaches. Streamer and stonefly nymph patterns remain effective when presented in deeper runs and holes where you can get to them..

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage Snow Water Equivalent (SWE) report of yesterday, March 11th, are below. The data give a grim picture of current SWE in the area.

Headlines:  

• More low temperature records were set yesterday, which started out with the coldest minimum in over five years.
• Water-year precipitation dropped to 82% of average, while SWE stayed at 68% of average.
• Reservoir fill rates were even lower yesterday than on Wednesday. Island Park is 86% full, compared with 79% full on average.
• After today, temperature and precipitation are expected to be near climatic normal for the next week or two.

Details:

Yesterday morning was the coldest in the watershed since December 17, 2016. In the 12-station 1989-2021 record I use for the whole watershed, yesterday’s minimum, mean, and maximum temperatures all set new records for the date. The low of -16.5 degrees F broke the old record of -3.2 degrees set in 2006. The daily maximum of 17.0 degrees broke the old record of 21.1 degrees, set in 2009. The daily mean was a whopping 27 degrees below average for the date and the second coldest day of the winter, behind January 1. That beat the old March-10 record by 9 degrees. Even at stations with very long periods of record, yesterday’s low temperature broke records for the date by wide margins. Idaho Falls set a new record low of -7 degrees, breaking the old record of 4 degrees, set in 1969.  

No precipitation was recorded yesterday, dropping the water-year total to 82% of average. Snow water equivalent (SWE) stayed at 68% of average. After another day of below-average temperatures, forecasts and long-range outlooks all call for temperature and precipitation near climatological averages. In the short term, light precipitation is expected on Sunday, with slightly heavier precipitation on Tuesday and Wednesday. The latest 7-day precipitation forecast looks a little better than it did yesterday, with 0.25-0.5 inch expected over most of the lower elevations of the watershed and up to 1.25 inches along the Teton crest. If realized (big if), that would be roughly average for this time of year.

As you might imagine, stream gages throughout the watershed remain affected by ice. The Island Park and Ashton gages are the only two that have provided reliable data all winter, and both show natural flow in the upper Henry’s Fork at around 75% of average.

Reservoir fill rates were even lower yesterday than they were on Wednesday. Island Park Reservoir is 86% full, compared with 79% on average, and the three reservoirs in the watershed are 87% full, compared with 84% full on average. The upper Snake River reservoir system is 50% full, compared with 74% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

HFF blog

Henry’s Fork, March 8th, 2022

Access to the lower river from the Ora bridge downstream to Chester is opening up. Increasing BWO activity during afternoons and midge hatches are providing some top water fishing that will increase as we move through this month. Life cycle patterns of these insects will produce. Streamer and giant stone fly nymphs will attract post-spawning rainbows almost anywhere in the river.

Fun Farm Bridge at Dusk

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage water status report of this morning follow.

Headlines:  

• Light snow late yesterday kept water-year precipitation at 82% of average and SWE at 67% of average.
• Island Park Reservoir continues to fill slowly but steadily at a rate of around 200-210 ac-ft/day.
• Light to moderate snow is forecast watershed-wide today.

Details:

Mean temperature yesterday was 12 degrees F below average and will stay that cold for the next several days. Snowfall arrived yesterday evening as forecast, but only a few hundredths of an inch were recorded through midnight, mostly in the valleys. Widespread snow is expected today, accompanied by strong winds and very cold temperatures this afternoon and evening. Unfortunately, water-equivalent totals now look to be lower than anticipated yesterday, a consistent pattern over the past two months. The heaviest snowfall appears to be headed to our south again, as happened a few days ago. More precipitation is expected Sunday and into next week, but this far out, forecast precipitation could easily disappear.

At an average outflow of 214 cfs, Island Park Reservoir gained 201 ac-ft yesterday and is 85% full, compared with 79% full on average. Yesterday I reported that the mean winter (December-February) outflow from Island Park Reservoir was 218 cfs, compared with 360 cfs on average. Winter flow in the Buffalo River was 200 cfs, compared with an average of 214 cfs, so total flow through Box Canyon—the most important variable determining survival of juvenile trout—was 418 cfs, 73% of average. However, natural stream flow was much lower than average this winter regardless of the effect of storage in Island Park and Henry’s Lake. In absence of the two reservoirs, stream flow through Box Canyon this winter would have been only 516 cfs. Thus, the actual stream flow through Box Canyon this winter was 81% of the river’s natural flow, not a bad outcome following last year’s very dry irrigation season.

:

Senior Scientist

Rob Van Kirk, Ph.D.

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

HFF blog

Henry’s Fork, February, 19th, 2022

Dr. Rob Van Kirk’s water report of February 18th for the the Henry’s Fork drainage is below.

Headlines:  

• Water-year total precipitation stayed at 90% of average yesterday, while SWE dropped to 75% of average.
• Island Park is 83% full, compared with 77% full on average, and the upper Snake River Reservoir system is 45% full, compared with 64% full on average.
• The new seasonal outlook issued yesterday calls for drought to persist through the spring.

Details:

Mean temperature yesterday was 3 degrees F below average. Light precipitation was recorded at scattered locations, averaging 0.04 inch over the watershed. The water-year total stayed at 90% of average, but snow water equivalent (SWE) dropped to 75% of average. Current SWE is fifth lowest for the date in the 1989-2021 record, ahead of 1991, 1994, 2001, and 2010. April-1 SWE in those other four years ranged from 59% of average in 2001 to 85% of average in 1991.

All signs point to a similar outcome this year. After light to moderate snow on Sunday and Monday, very cold but dry conditions are expected next week and into the first week of March. The new seasonal outlook for March-May issued yesterday calls for drought conditions to persist. Our best-case scenario this year is a snowpack about like last year’s, in the 80-85% of average range. However, when combined with near record-low stream base flows this winter, water supply will definitely be lower than it was last year.  

Reservoirs continue to fill slowly but steadily. Island Park Reservoir is 83% full, compared with 77% full on average. Henry’s Lake is 90% full and very close to average. The upper Snake River reservoir system is 45% full, compared with 64% full on average. Even with above-average precipitation over the remainder of the winter, snowpack will not be sufficient to fill the upper Snake system.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

Henry’s Fork, February 25th, 2022

Excerpts from Dr. Rob Van Kirk’s February 24th Henry’s Fork water supply report are below.

Headlines:  

• A very dry week dropped water-year precipitation to 92% of average and SWE to 77% of average.
• Since Wednesday’s reduction in outflow, Island Park Reservoir fill rate has averaged 233 ac-ft/day, up from 150-190 ac-ft/day for most of the last five weeks.
• Weather forecasts have been highly variable from day to day over the past week but are certain of widespread snowfall tonight and tomorrow and chances of precipitation over the next week.

Details:

Mean temperature last week was 2 degrees F above average, and yesterday’s watershed-average maximum temperature of 39 degrees was the warmest since December 4. Last week was as dry as any in the current dry spell, now in its sixth week. Precipitation was recorded at only four of the 12 stations, amounting to only a few hundredths of one inch when averaged across the watershed. This dropped water-year precipitation from 96% of average last week to 92% of average this morning. In the bigger picture, the three-year running average precipitation, which is a good measure of overall drought conditions at a scale relevant for ecological process and long-term water supply, dropped from just a fraction of an inch below average in early January to 2.5 inches below average over the past five weeks.

The figures for snow water equivalent (SWE) are even worse. Net accumulation of new SWE over the week was 0, dropping SWE from 81% of average to 77% of average over the week. If we have average SWE accumulation between now and early April, this year’s snowpack will end up around 20% below average. At best, we could barely hit average if the remainder of the winter is extremely wet. In the 1989-2021 record, only two years—2011 and 1989—were wet enough between February 14 and April 8 to push this year’s snowpack up to average. That’s only two observed scenarios in the past 33 years, giving us roughly a 6% chance of getting to average in the modern climatic regime.

Weather forecasts and climate outlooks at all scales have not only performed poorly over the past month but have become highly variable from day to day over the past week. Forecasts for this week went from wet a week ago to dry by the end of last week to wet again this morning. Widespread precipitation is expected tonight and tomorrow, followed by moderate chances over the remainder of the week. More widespread precipitation is expected again next weekend. The current 7-day quantitative precipitation forecast is the best it has looked in over a month, calling for widespread amounts of 0.25 inch in the valleys to over 1 inch along the Wyoming border. However, confidence is low past Wednesday of this week. Nonetheless, we will see at least some change from persistent high pressure.  

Since Wednesday’s 20-cfs reduction in outflow from Island Park Reservoir, fill rate has averaged 233 ac-ft/day, up from 150-190 ac-ft/day over most of the past five weeks. The reservoir is 82% full, compared with 76% on average. If precipitation over the next week materializes as predicted, gain will be enhanced by about 50 ac-ft/day due to direct precipitation. Outflow has averaged 217 cfs since December 1.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

Henry’s Fork, February 8th, 2022

During nicer days midge activity is dense enough to provide some good top water fishing up and down the river, but getting to the river can be a problem in many places.

Dr. Rob Van Kirk’s report for today, February 8th, on snow and water conditions in the upper Henry’s Fork drainage are given below.

Headlines:  

• Water-year precipitation stayed at 96% of average yesterday, but SWE dropped to 80% of average.
• Island Park Reservoir gained 187 ac-ft yesterday, a little higher than it was 7-10 days ago but well below the rates of 250-350 ac-ft/day we saw during periods of heavy precipitation in December.
• Natural flow in the upper Henry’s Fork sub-watershed remains around 75% of average.
• Dry weather will continue until at least next Monday.

Details:

Mean temperature yesterday was 2 degrees F below average. No precipitation was recorded, leaving the water-year total at 96% of average. Snow water equivalent (SWE) dropped to 80% of average. Forecasts are hinting at precipitation next Monday, but we’ve seen such forecasts fall apart week after week over the past month.

Most stream gages in the watershed remain affected by ice, but the Island Park and Ashton gages on the Henry’s Fork show that natural flow in the upper Henry’s Fork watershed is around 75% of average, where it has been since late last summer.

Island Park Reservoir gained 187 ac-ft yesterday, higher than what we saw 7-10 days ago but nowhere near the rates of 250-350 ac-ft/day observed back in December during periods of precipitation. Since December 1, outflow has average 218 cfs, and fill has averaged 247 ac-ft/day. The reservoir is 81% full, compared with 75% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

Henry’s Fork, January 11th, 2022

Excerpts From Dr. Rob Van Kirk’s Henry’s Fork drainage water report for today are below.

Headlines:  

• Dry conditions have set in for around 10 days, during which water-year precipitation and SWE will fall with respect to average and reservoir fill rates will drop.
• Water-year precipitation is 116% of average, and SWE is 104% of average this morning.
• At a mean outflow of 218 cfs, Island Park Reservoir gained 227 ac-ft yesterday, compared with 265 ac-ft/day needed over the remainder of the winter to reach the April-1 target.

Details:

Mean temperature yesterday was 5 degrees F below average. Water-year precipitation dropped to 116% of average and SWE to 104% of average. Dry conditions are expected to last into the beginning of next week, by which time SWE will be 5-7% below average again.

At a mean outflow of 218 cfs, Island Park Reservoir gained 227 ac-ft yesterday and is 77% full, compared with 72% full on average. Since December 1, the reservoir has filled at 295 ac-ft/day, ahead of the 265 ac-ft/day needed over the remainder of the winter to reach the April-1 target 126,000 ac-ft (93% full). If precipitation in Island Park stays above-average for the rest of the winter, as is expected, the reservoir will reach the April-1 target at an average outflow of 216 cfs, within measurement error of current outflow. If reservoir fill rates deviate too much one way or the other from forecasts, outflow adjustments will need to be made later in the winter to keep the reservoir on track to meet the target.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

HFF blog

Henry’s Fork, January 8th, 2022

Dr. Rob Van Kirk’s Henry’s Fork drainage snow water equivalent (SWE) report of January 7th is below. It’s data suggests that we are off to a good start in accumulating snowfall in the drainage.

DATA NOTE: The Grassy Lake and Black Bear SnoTel stations have been reporting incomplete data for the past few days. I have estimated these using available data, but I anticipate adjustments once complete data are reported again. Those adjustments may affect precipitation totals and temperatures.

 Headlines:  

• Precipitation yesterday was the heaviest since December 23, increasing the water year total up to 119% of average and SWE up to 111% of average.
• Reservoir fill rates increased again yesterday due to the heavy precipitation, especially at Henry’s Lake and Grassy Lake.
• At an outflow of 222 cfs, Island Park Reservoir gained 309 ac-ft yesterday, compared with 261 ac-ft/day needed over the remainder of the winter to reach the April-1 target.
• Warm, wet weather is expected to continue today before a lengthy dry spell sets in.

Details:

Yesterday was the warmest and wettest day since December 23. Temperature was 7 degrees above average, and precipitation averaged 0.54 inch across the watershed. The Teton and Fall River subwatersheds were favored. Water-year precipitation is now 19% above average for the watershed and above average at all 12 stations. New snow water equivalent (SWE) accumulations ranged from 0.2 inches at Crab Creek to 2.3 inches at Grand Targhee. This increased the watershed-total SWE to 11% above average. SWE is above average at all SnoTel stations except Grand Targhee, which is at 98% of average.

Warm, windy, and wet conditions are expected today before an extended dry period sets in. Temperatures in inversion-prone valley areas will be average to below average, while elevations above inversions will see above-average temperatures next week.

In response to precipitation and warmer temperatures, total gain in the watershed’s three reservoirs increased from 220 ac-ft on Monday to 482 ac-ft yesterday. The increases were most notable at Grassy Lake and Henry’s Lake. Grassy Lake gained only 11 ac-ft on Monday but 33 ac-ft yesterday and is 67% full, compared with 76% full on average. Henry’s Lake actually lost some volume last weekend but gained 140 ac-ft yesterday. Henry’s Lake is 88% full, compared with 89% full on average.

At an outflow of 222 cfs, Island Park Reservoir gained 309 ac-ft yesterday. Average fill rate since December 1 is 307 ac-ft/day, compared with 261 ac-ft/day needed the rest of the winter to meet the April-1 target. The reservoir is 77% full, compared with 71% full on average. Together, the three reservoirs in the Henry’s Fork watershed are 81% full, compared with 78% on average. The upper Snake River reservoir system as a whole is 36% full, compared with 56% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

Henry’s Fork, December 14th, 2021

Anywhere you try the Henry’s Fork open to fishing, winter conditions will be your companion. Happy Holidays!

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork watershed report released yesterday follow.

Headlines:  

• Yesterday was warm again but the wettest since November 19, moving water-year precipitation up to 94% of average.
• New SWE accumulation was heavy yesterday, especially in the upper Henry’s Fork, increasing watershed-averaged SWE to 68% of average.
• Natural flow increased to 83% of average in response to warmer temperatures and low-elevation rain.
• Island Park Reservoir gained 625 ac-ft yesterday, over half of which was due to direct precipitation, and is 71% full, compared with 67% full on average.

Details:

Mean temperature yesterday was 9 degrees above average, and moderate to heavy precipitation fell across the watershed. Low elevations received a mix of rain and wet snow, while all SnoTel stations accumulated new snow water equivalent (SWE). Precipitation totals ranged from 0.02 inch at Alta to 1.3 inches at White Elephant. As has been the case recently, the upper Henry’s Fork and Fall River subwatersheds received the most precipitation. The watershed average was 0.43 inches, the highest one-day total since November 19. Water-year total precipitation increased from 91% of average yesterday to 94% of average this morning. New SWE totals ranged from 0.2 inch at Pine Creek Pass to 1.3 inches at White Elephant and averaged 0.7 inch over the nine SnoTel stations. This improved SWE from 62% of average yesterday to 68% of average today. Temperatures will drop back to average tonight and stay near to slightly below average for the next week or two. Above-average precipitation is expected to continue for the foreseeable future.

Watershed-total natural streamflow increased in response to yesterday’s warm temperatures and low-elevation rain. Natural flow was 17% below average yesterday. Cumulative natural flow for the water year to date is still 23% below average.

USGS measured outflow from Island Park Reservoir at 213 cfs yesterday afternoon. Updated outflow data are around 7% lower than I have been reporting for the past month or so. The reservoir gained 625 ac-ft yesterday, over half of which was from direct precipitation on the reservoir surface. Since December 1, reservoir outflow has averaged 205 cfs, and the reservoir has gained 325 ac-ft/day. An average fill rate of 270 ac-ft/day is needed to reach the April-1 reservoir target of 93% full. The reservoir is currently 71% full, compared with 67% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

HFF blog